1. Field of the Invention
This invention relates to line scanners and methods for scanning multiple synchronized scan lines using scanners having high duty cycles that conserve illumination source energy.
2. Description of Related Art
Many devices such as printers, inspection devices, and medical equipment contain line scanners. For example, a typical laser printer contains a line scanner that scans a digitally modulated laser beam to form an image on a media such as paper, film, or plates, and in the medical industry, a line scanner in an X-ray machine scans an X-ray beam through a patient or sample to form lines of an image. A typical line scanner includes a scanning element such as a polygon mirror or a holographic disc on which an input beam from a laser or other light source is incident. The scanning element redirects the input beam, for example, by reflection or diffraction, to project a scan beam. Movement such as rotation or oscillation of the scanning element moves or scans the scan beam along a scan line in an image area. Line scanners often include pre-scan and post-scan optical systems that adjust the focus or collimation of the input and scan beams as well the linearity of the scan line and the uniformity of scan rate.
An important property of a line scanner is the duty cycle which is defined by the ratio of the time that the scan beam is imaging to the total operating time. Generally, a line scanner has a periodic motion that includes a dead time during which the line scanner is not directing the scan beam toward the image area. Thus, duty cycles for line scanners are generally less than 100% and typically range from about 20% to 70%. The duty cycle of a line scanner can limit the useful output illumination per unit of input power from a laser or other light source. Accordingly, a low duty cycle line scanner requires either a higher power light source, more efficient optics, or a slower scan rate to provide a fixed amount of useful illuminating energy density. Accordingly, line scanners having high duty cycles are sought.
Another important property of line scanners is the imaging area or scan line length. Conventionally, creating long scan lines to cover a large imaging area requires either a large line scanner or multiple smaller line scanners operated in series. Using multiple line scanners (or scan heads) to create a long scan line has a number of advantages including the ability to design smaller or more compact systems with less expensive scanning heads. However, multiple-head systems often require elaborate and expensive line-connecting optics or electronics to control the relative positions of the scan lines from the various scan heads and to synchronize the independently operating scan heads. This is due in part to the lack of synchronization of the motion of the light sources and the scanning optics in the multiple scan heads. U.S. Pat. No. 5,654,817 describes a system employing multiple scan heads for large area imaging. Line scanners that are compact and do not require expensive line connecting optics or electronics are desired.